users: Avoid error when user's groups cannot be parsed

Add log warnings to help debug if there is a related issue.

May help #1834.

Tested:
- Run action command with valid and invalid username. Warning is printed for
  invalid username.
- Modify the output to remove '='. Warning is printed instead of exception.
- Ensure that warnings messages are output to stderr and not stdout.
- On frontpage.py change the call to get user groups and ensure that that output
  warning messages are not parsed groups.

Signed-off-by: James Valleroy <jvalleroy@mailbox.org>
[sunil: Drop module logger as root logger is at use]
[sunil: Use %s formatting for logging API]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

actions/users

@@ -5,6 +5,7 @@ Configuration helper for the LDAP user directory
 """
 
 import argparse
+import logging
 import os
 import re
 import shutil
@@ -12,6 +13,7 @@ import subprocess
 import sys
 
 import augeas
+
 from plinth import action_utils
 
 ACCESS_CONF = '/etc/security/access.conf'
@@ -200,8 +202,8 @@ def configure_ldapscripts():
     # modify a copy of the config file
     shutil.copy('/etc/ldapscripts/ldapscripts.conf', LDAPSCRIPTS_CONF)
 
-    aug = augeas.Augeas(
-        flags=augeas.Augeas.NO_LOAD + augeas.Augeas.NO_MODL_AUTOLOAD)
+    aug = augeas.Augeas(flags=augeas.Augeas.NO_LOAD +
+                        augeas.Augeas.NO_MODL_AUTOLOAD)
     aug.set('/augeas/load/Shellvars/lens', 'Shellvars.lns')
     aug.set('/augeas/load/Shellvars/incl[last() + 1]', LDAPSCRIPTS_CONF)
     aug.load()
@@ -325,13 +327,20 @@ def get_user_groups(username):
     output = process.stdout.decode().strip()
     if output:
         groups_part = output.split(' ')[2]
-        groups = groups_part.split('=')[1]
+        try:
+            groups = groups_part.split('=')[1]
+        except IndexError:
+            logging.warning('Could not read groups for user %s: \n%s',
+                            username, output)
+            return []
+
         group_names = [
             user.strip('()') for user in re.findall(r'\(.*?\)', groups)
         ]
         group_names.remove('users')
         return group_names
 
+    logging.warning('User %s not found in LDAP', username)
     return []